The present invention relates to a method for producing a field emission display (FED), and more particularly to a method for forming field emitters by depositing a field emitter material from a carrier gas in a space formed between an anode structure and a cathode structure.
A flat display screen is an electronic display composed of a large area filled with individual pixels. These pixels can be arranged side-by-side in the form of a two-dimensional matrix, such as a checkerboard pattern. Various types of flat panel displays, such as electroluminescence, AC-plasma, DC-plasma and field emission display screens, can be produced by several processes known in the art.
The present invention relates in particular to field emission display screens, which have a cathode and an anode structure arranged with a relatively small spacing therebetween. Electrons are emitted from the cathode by applying an electric field between the cathode and anode and propelled towards the anode. To facilitate electron emission, the electrodes of the cathode structures are covered at least in part with a field emitter made of a material with advantageous field emission properties.
The anode structure is transparent and coated with a luminescent material, such as a phosphor, which lights up at those locations that are struck by the emitted electrons.
Conventional field emission display screens are presently produced by manufacturing complete anode and cathode structures on flat substrates, including the electrodes, as well as the layer of luminescent material applied to the anode structure and the field emitters applied to the cathode structure. The anode and cathode structure are subsequently placed against each other and sealingly connected with one another along their lateral edges in a gas-tight manner, optionally by interposing a spacer and/or a grid electrode. In a last step, the space between the cathode and anode structure is evacuated so that the electrodes can travel essentially unimpededly from the cathode to the anode.
The cathode structure, and more particularly the surface of the field emitters provided on the cathode structure, has to be kept in a clean environment, i.e., kept free of dust particles, between the time of manufacture and the time when the cathode structure is attached to the anode structure. Dust particles that settle on a field emitter surface, can prevent electrons emitted from the cathode in the region of these particles from reaching the anode, causing the display to malfunction in that region. If dust particles residing on the surface of the field emitters are not detected in due time and removed before the anode and cathode structures are assembled, then the FED will exhibit defects and become unusable and may have to be scrapped. The manufacture of FED with a high yield therefore tends to require clean rooms, which increases the complexity and cost of their manufacture.
European Pat. No. EP 0 800 198 A discloses a method for producing a field emission display with a base plate and a cover plate, a phosphor layer and a substrate with an electrode structure. According to the disclosed method, a carbon-containing field emitting layer is deposited on the substrate from a carbon-containing gas, such as acetylene, in the space between the base plate and the cover plate of the display.
In a first activation step, the electrode structure is formed on the substrate, while the carbon-containing film is formed in a second activation step. Any residual organic materials in the display are removed in a stabilization step. In a final finishing step, additional organic material is introduced in the interior space of the display to slow degradation of the field-emitting carbon-containing layer during the operating life of the display. By introducing organic materials, the atoms that are removed from the field-emitting layer during activation of individual pixels are replaced by the carbon atoms in the vacuum. This essentially represents an equilibrium process, with the average absorption time of the organic materials advantageously in the range of the activation frequency of the display, which for typical computer displays is approximately 60 Hz.
It would therefore be desirable and advantageous to provide a less complex process for producing field emission display screens which can be manufactured in an environment that does not require stringent clean room conditions.
According to one aspect of the invention, the electrodes of the anode structure are disposed on a first substrate and the electrodes of the cathode structure are disposed on a second substrate. The luminescent layer formed of the luminescent material is then deposited over the electrodes of the anode structure, whereafter the cathode structure and the anode structure with the luminescent layer are joined while facing each another so as to form a gas-tight seal along their lateral edges, except for a gas inlet port and a gas outlet port. After the cathode and anode structure are sealingly connected with one another in a gas-tight manner, a carrier gas is introduced through the gas inlet port between the cathode and the anode structure to deposit the field emitters on the electrodes of the cathode structure.
The process according to the present invention essentially eliminates deposition of dust particles on the completed field emitter surfaces, because the field emitters are formed only after the surface of the cathode structure is hermetically sealed from the environment by the gas-tight connection with the anode structure.
According to another feature of the present invention, the electrodes can be raised to the temperature required for depositing the field emitter material on the electrodes by inductive heating. In this way, only the electrodes are heated, whereas all other elements of the FED are kept at a lower temperature which is insufficient for depositing the field emitter material, thereby effectively eliminating the formation of unwanted field emitter layers on FED elements other than the electrodes of the cathode structure.
According to another feature of the present invention, the field emitter material can be deposited on the electrodes by heating the electrodes with an applied current. This approach also prevents the formation of unwanted field emitter layers on the components other than the cathode structure electrodes and has the additional advantage over the first heating method that this heating method does not require additional components (except for a voltage or current source) since the electrodes themselves operate as heaters.
According to another embodiment of the invention, the field emitters can be in the form of carbon-containing layers produced by introducing a carbon-containing carrier gas between the cathode structure and the anode structure. Carbon-containing layers have relatively good field emission properties which makes them suitable for the formation of reliable field emitters. Moreover, deposition conditions for carbon-containing layers are known in the art and, more importantly, such layers can also be produced inside the relatively narrow space between the cathode structure and the anode structure.
According to yet another feature of the present invention, the carbon-containing layers can have the form of nanotube layers. Carbon nanotubes are particularly efficient field emitters, so that an FED produced in this manner can operate reliably for long periods of time at low addressing voltages.